1. Field of the Invention.
The present invention relates to improved methods and compositions for sealing subterranean zones to prevent the uncontrolled flow of fluids into or from the zones.
2. Description of the Prior Art.
In the drilling of oil and gas wells using the rotary drilling method, drilling fluid is circulated through the drill string and drill bit and then back to the surface by way of the well bore being drilled. The drilling fluid maintains hydrostatic pressure on the subterranean zones through which the well bore is drilled and circulates cuttings out of the well bore. During such drilling, highly permeable zones are often encountered whereby the drilling fluid circulation is lost and drilling operations must be terminated while remedial steps are taken. Also, when a subterranean zone is penetrated containing fluids under pressure which exceeds the hydrostatic pressure exerted on the zone by the drilling fluid, formation fluid cross-flows and/or underground blow-outs can and often do occur.
Heretofore, a variety of sealing compositions have been developed and used for combating lost circulation, cross-flow and underground blow-out problems. However, such compositions have often been unsuccessful due to delayed and inadequate viscosity development by the compositions. For example, a variety of cement compositions have been used in attempts to stop lost circulation. The lost circulation is usually the result of encountering weak permeable subterranean zones that contain natural fractures or are fractured by drilling fluid pressures and rapidly break down. When a conventional cement or other slow setting composition is squeezed into the zone, the delay in developing high viscosity allows the composition to be diluted and displaced deeply into the zone whereby it bypasses the highly permeable fractures, vugs and the like causing the lost circulation. The same type of problem often occurs when cross-linked hydrated gels and other similar plugging compositions are utilized.
Thus, there are needs for improved methods and sealing compositions for sealing subterranean zones that develop high viscosity in a few seconds or minutes instead of the longer times heretofore required.
Improved methods and sealing compositions for sealing subterranean zones are provided which overcome the deficiencies of the prior art and meet the needs described above. The methods are basically comprised of the steps of introducing a sealing composition into the subterranean zone comprised of a hydraulic cement, sufficient water to form a slurry and a rapidly acting thixotropy providing gelling agent comprising at least one compound selected from the group of compounds of the formula MaMb(SO4)2. xH2O, wherein
Ma is a monovalent metal ion having an oxidation state of I,
Mb is a trivalent metal ion having an oxidation state of III, and
x is the number of moles of water of crystallization in the range of from 0 to 24. Thereafter, the sealing composition is allowed to remain at rest in the zone whereby the sealing composition rapidly gels and then sets into a hard impermeable mass therein. The rapid gelling of the cement composition prevents it from being diluted and displaced into the zone during the time required for the cement composition to set.
The compositions of the invention are basically comprised of a hydraulic cement, sufficient water to form a slurry and a rapidly acting thixotropy providing gelling agent as described above. The sealing compositions can also include a viscosity reducing agent and one or more additives such as set retarding agents, fluid loss control agents, set accelerating agents, dispersing agents and formation conditioning agents.
It is, therefore, a general object of the present invention to provide improved methods and compositions for sealing permeable subterranean zones penetrated by well bores.
Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of preferred embodiments which follows.
As mentioned above, drilling fluid circulation is often lost during the drilling of a well bore which requires the termination of the drilling and the implementation of remedial procedures which are often of long duration and high costs. The remedial procedures have heretofore involved the placement of hardenable compositions such as cement compositions, cross-linked stiff gels and the like in the permeable lost circulation zone. However, because such compositions have heretofore required considerable time to harden or gel and are easily washed out, successful sealing of the lost circulation zone often does not take place. In addition to drilling fluid lost circulation zones, zones containing pressurized fluids can be encountered which cause gas, oil or water cross-flows that dilute and wash away sealing compositions. Also, underground blow-outs at low to high formation fluid flow rates can take place.
The present invention provides improved methods and sealing compositions for sealing permeable subterranean zones to thereby prevent the uncontrolled flow of fluids into or from the zones. When a sealing composition of this invention is placed in a subterranean zone to be sealed, the sealing composition rapidly forms a gel which prevents it from being washed out of the zone and allows the sealing composition to harden into an impermeable sealing mass therein.
The methods of this invention for sealing a permeable subterranean zone penetrated by a well bore are comprised of the steps of introducing a sealing composition into the subterranean zone which has thixotropic properties and when at rest rapidly forms a gel. Thus, the sealing composition can be pumped or displaced into the zone to be sealed by way of a well bore penetrating the zone without gelling. Once the sealing composition reaches the subterranean zone to be sealed, it is allowed to remain at rest therein whereby it rapidly gels and resists wash-out for the additional time period required for the sealing composition to set into a hard impermeable mass therein.
The sealing compositions of this invention are basically comprised of a hydraulic cement, sufficient water to form a slurry and a rapidly acting thixotropy providing gelling agent comprising at least one compound selected from the group of compounds of the formula MaMb(SO4)2. xH2O, wherein
Ma is a monovalent metal ion having an oxidation state of I,
Mb is a trivalent metal ion having an oxidation state of III, and
x is the number of moles of water of crystallization in the range of from 0 to 24. As mentioned, once the sealing composition is placed in the zone to be sealed and is allowed to rest, the sealing composition rapidly gels whereby it is not washed out of the zone while it sets into a hard impermeable mass.
A variety of hydraulic cements can be utilized in the sealing compositions of this invention including those comprised of calcium, aluminum, silicon, oxygen and/or sulfur which set and harden by reaction with water. Such hydraulic cements include Portland cements, pozzolana cements, gypsum cements, slag cements, silica cements and aluminous cements. Portland or the equivalent cements are generally preferred for use in accordance with the present invention. Portland cements of the types defined and described in the API Specification For Materials And Testing For Well Cements, API Specification 10, 5th Edition, dated Jul. 1, 1990 of the American Petroleum Institute are particularly suitable. Preferred API Portland cements include Classes A, B, C, G and H with API Classes G and H being the most preferred. The hydraulic cements utilized in the present invention can also include additives such as fly ash for extending the cement composition and prehydrated bentonite for providing viscosity to the cement composition.
The water utilized in the cement compositions of this invention can be fresh water, unsaturated aqueous salt solutions including brines or seawater or saturated aqueous salt solutions. The water is generally present in the sealing compositions of this invention in an amount in the range of from about 30% to about 160% by weight of hydraulic cement in the compositions, more preferably in an amount in the range of from about 37% to about 56%.
As mentioned, the rapidly acting thixotropy providing gelling agents which are useful in this invention include at least one compound selected from the group of compounds of the formula MaMb(SO4)2. xH2O, wherein
Ma is a monovalent metal ion having an oxidation state of I,
Mb is a trivalent metal ion having an oxidation state of III, and
x is the number of moles of water of crystallization in the range of from 0 to 24. Examples of the monovalent metal ion Ma include, but are not limited to, sodium, potassium and ammonium. Examples of the trivalent metal Mb include, but are not limited to, aluminum, titanium, chromium, manganese, iron, cobalt, gallium, indium, rhodium and iridium. Preferred such gelling agents are compounds of the above formula wherein x is 12. Examples of preferred gelling agents include, but are not limited to, aluminum potassium sulfate, aluminum sodium sulfate, ferrous ammonium sulfate, ferric ammonium sulfate, aluminum ammonium sulfate and mixtures thereof. Of these, aluminum potassium sulfate is the most preferred.
As also mentioned, when a rapidly acting thixotropy providing gelling agent of this invention is combined with hydraulic cement and water to form a sealing composition of this invention the composition has thixotropic properties, i.e., the sealing composition is a liquid when it is being pumped but quickly and completely gels when at rest. The gelling agent is preferably included in the sealing compositions of this invention in an amount in the range of from about 0.5% to about 5% by weight of the hydraulic cement in the composition, more preferably in an amount in the range of from about 1% to about 3% and most preferably about 2%.
As also mentioned above, the sealing compositions can also include a viscosity reducing agent to facilitate the pumping of the compositions into the zones to be sealed. Suitable viscosity reducing agents that can be used include sodium sulfate, potassium sulfate, sulfonated naphthalene-formaldehyde condensate, sulfonated acetone-formaldehyde condensate and sulfonated melamine-formaldehyde condensate. Of these, sodium sulfate is preferred. When utilized, the viscosity reducing agent is included in the sealing compositions of this invention in an amount in the range of from about 0.25% to about 5% by weight of the hydraulic cement in the compositions, more preferably in an amount in the range of from about 0.3% to about 3%, and most preferably about 0.5%.
As will be understood, the sealing compositions of this invention can include a variety of additives for improving or changing the properties of the compositions. Examples of such additives include, but are not limited to, set retarding agents, fluid loss control agents, set accelerating agents, dispersing agents and formation conditioning agents.
Set retarding agents are included in the cement compositions when it is necessary to extend the time in which the cement compositions can be pumped so that they will not thicken or set prior to being placed in the desired subterranean zone. Examples of set retarding agents which can be used include, but are not limited to, lignosulfonates such as calcium and sodium lignosulfonate, organic acids such as tartaric acid and gluconic acid, copolymers and others. The proper amount of set retarding agent required for particular conditions can be determined by conducting a xe2x80x9cthickening time testxe2x80x9d for the particular retarder and cement composition. Such tests are described in the API Specification 10 mentioned above. A particularly preferred set retarder for use in accordance with the present invention is a copolymer or copolymer salt of 2-acrylamido-2-methylpropane sulfonic acid and acrylic acid. When used, a set retarder is included in the sealing composition of this invention in an amount in the range of from about 0.1% to about 2% by weight of hydraulic cement in the composition.
Examples of fluid loss control agents which can be used include, but are not limited to, cellulose derivatives, modified polysaccharides, polyacrylamides, guar gum derivatives, 2-acrylamido-2-methylpropane sulfonic acid copolymers, polyethyleneimine and the like.
An example of a dispersing agent which can be utilized is comprised of the condensation polymer product of an aliphatic ketone, an aliphatic aldehyde and a compound which introduces acid groups into the polymer, e.g., sodium bisulfite. Such a dispersant is described in U.S. Pat. No. 4,557,763 to George et al. on Dec. 10, 1985.
Examples of set accelerating agents which can be utilized include, but are not limited to, calcium chloride, zinc formate and triethanolamine, and examples of formation conditioning agents include, but are not limited to, potassium chloride and sodium chloride.
A method of the present invention for sealing a permeable subterranean zone penetrated by a well bore to prevent the uncontrolled flow of fluids into or from the zone comprises the steps of: (a) introducing a sealing composition into the subterranean zone comprised of a hydraulic cement, sufficient water to form a slurry and a rapidly acting thixotropy providing gelling agent comprised of at least one compound selected from the group of compounds of the formula MaMb(SO4)2. xH2O, wherein
Ma is a monovalent metal ion having an oxidation state of I,
Mb is a trivalent metal ion having an oxidation state of III, and
x is the number of moles of water of crystallization in the range of from 0 to 24; and (b) allowing said sealing composition to remain at rest in said zone whereby said sealing composition rapidly gels and then sets into a hard impermeable mass therein.
A more preferred method of the present invention for sealing a permeable subterranean zone penetrated by a well bore to prevent the uncontrolled flow of fluids into or from the zone comprises the steps of: (a) introducing a sealing composition into the subterranean zone comprised of a hydraulic cement, sufficient water to form a slurry and at least one rapidly acting thixotropy providing gelling agent selected from the group of aluminum potassium sulfate, aluminum sodium sulfate, ferrous ammonium sulfate, ferric ammonium sulfate and aluminum ammonium sulfate; and (b) allowing said sealing composition to remain at rest in said zone whereby said sealing composition rapidly gels and then sets into a hard impermeable mass therein.
A preferred subterranean formation sealing composition of this invention is comprised of: a hydraulic cement; sufficient water to form a slurry; and a rapidly acting thixotropy providing gelling agent comprised of at least one compound selected from the group of compounds of the formula MaMb(SO4)2. xH2O, wherein
Ma is a monovalent metal ion having an oxidation state of I,
Mb is a trivalent metal ion having an oxidation state of III, and
x is the number of moles of water of crystallization in the range of from 0 to 24.
A more preferred subterranean formation sealing composition of this invention is comprised of: Portland cement; sufficient water to form a pumpable slurry; a rapidly acting thixotropic providing gelling agent comprised of aluminum potassium sulfate present in an amount of about 2% by weight of cement in the composition; and a viscosity reducing agent comprised of sodium sulfate present in an amount of about 0.5% by weight of cement in the composition.